1. Join a public lobby
2.Start a mission (Meth'd up, as usual...)
3.Park the car you want to dupe so that you can teleport inside
4.Bring a street car to the garage door using the interaction menu, not the phone
5.Shoot the hood till it burns
6.Enter the car while you keep pressing RT, and spamming Y until you are outside the car.

Keeping RT pressed while entering the car will keep the driver's door open (thus making timing easier) and start the map blinking process. You must release RT before getting outside the car.

7.Once outside, if you keep spamming Y you should be teleported into the car you want to dupe (don't spam Y too much)

NOT MY GLITCH JUST COPIED IT!